Here

Killing or wounding treacherously a combatant adversary;

And more information on what this means, relevant part:

The perpetrator invited the confidence or belief of one or more persons that they were entitled to, or were obliged to accord, protection under rules of international law applicable in armed conflict

By pretending to be civilians (not to mention killing an unarmed person in FUCKING HOSPITAL) they are committing a war crime. The victim’s uniform is irrelevant